1. Field of the Invention
The present invention is directed to an evaporative gas conditioning system for conditioning gases resulting from industrial processes. More particularly, the present invention is directed to a method and apparatus for controlling an evaporative gas conditioning system.
2. Description of the Related Art
Evaporative gas conditioning systems for cooling and conditioning hot gases produced by industrial processes are well known. Evaporative gas conditioning systems are most commonly utilized upstream from pollution control equipment, such as an electrostatic precipitator or a baghouse, so as to condition, or prepare, the industrial gases for the pollution control equipment. As hot, particulate laden gas resulting from an industrial process, such as a kiln or furnace operation, is introduced into a conditioning chamber or duct of an evaporative gas conditioning system, the physical and chemical properties of the gas are operated upon for the benefit of the air pollution control equipment located downstream. Particularly, conditioning of the gas typically involves altering the temperature, moisture content, resistivity of the dust and chemical composition of the gas.
Conventionally, industrial gases introduced into a conditioning chamber of an evaporative gas conditioning system are conditioned by spraying water into the gas stream. Spraying of water into the gas stream results in a reduction in the temperature of the gas, a reduction in the gas volume handled by the downstream air pollution control equipment, a reduction in resistivity of particulate located within the gas stream, and an increase in the overall moisture content of the gas.
Evaporative gas conditioning systems conventionally fall into one of two categories--single fluid systems and dual fluid systems. Single fluid evaporative gas conditioning systems are characterized by the introduction of only high-pressure liquid (water) for conditioning purposes. In contrast, dual fluid evaporative gas conditioning systems introduce both water and air at lower pressure into the gas stream. In either type of system, water is introduced into the gas stream by nozzles, which output a spray in the form of water droplets. Typically, single fluid evaporative gas conditioning systems are characterized by water droplets of 350 microns or more in size, while dual fluid systems output much smaller water droplets, often on the order of 50-136 microns. Dual fluid evaporative gas conditioning systems therefore typically utilize less water than single fluid systems, and are able to operate with smaller conditioning towers. Moreover, one advantage of a dual fluid evaporative gas conditioning system is that, since the water droplets are smaller, it is more likely that they will evaporate before passing through the conditioning chamber, thus preventing downstream water damage.
In the past, dual fluid evaporative gas conditioning systems were controlled utilizing a control loop known as proportional integral derivative, or PID. As will be appreciated by those with skill in the art, a primary drawback with the PID approach is, when introducing water into the conditioning chamber to lower the temperature of the gas from a sensed temperature to a desired temperature, it is common to overshoot the desired temperature, which thus requires a correction. However, when undertaking correction, it is not uncommon for control systems based upon a PID loop to over correct, and so forth, resulting in oscillation of temperature above and below the desired temperature. The resulting output from the conditioning chamber is thus unstable, causing performance problems and possible equipment damage downstream from the conditioning system. Additionally, a specific drawback of the PID approach is the inability of the system to be sensitive to on-going conditions in the system, and to respond as necessary to those on-going conditions. More specifically, with the PID approach, the nozzles utilized to introduce water droplets into the conditioning chamber cannot be adequately controlled to adjust the rate at which water is introduced into the chamber, thus resulting in the oscillation problems described.
In response to the limitations of the PID loop approach, the present applicant developed an evaporative gas conditioning system which took a different approach. In that system, water was introduced into the conditioning chamber at a first maximum rate when the temperature of the gas in the chamber was more than a selected amount away from the desired temperature. Upon reaching a predetermined threshold temperature, the rate at which water was supplied to the conditioning chamber to cool gas was based upon a linear function. However, it has been determined that such a system has numerous drawbacks, is limited in its ability to efficiently respond to conditions in the conditioning chamber, and results in an unacceptable overall performance by the conditioning system.
An additional problem with conventional evaporative gas systems is their inflexibility to accommodate nozzles having different characteristics. In other words, in the design of a conventional evaporative gas conditioning system, once a nozzle is selected, the performance parameters of the system are designed around the characteristics of the selected nozzle. In the event that it becomes necessary or desirable to change the nozzles to a type having different characteristics at a later time, conventional systems require a substantial reconfiguration to accommodate the new nozzle type.
Accordingly, the need exists for a dual fluid evaporative gas conditioning system which is responsive to the on-going conditions within the conditioning chamber of the system. Additionally, the need exists for an evaporative gas conditioning system that does not have the temperature overshoot problems associated with utilization of the PID loop, and which overcomes the drawbacks of the linear approach taken in other prior art.
Additionally, the need exists for an evaporative gas conditioning system which is easily adjustable to utilized nozzles having different performance characteristics. The present invention overcomes the drawbacks of the prior art, and meets these and other needs.